Leather-like sheets made of a synthetic resin are already known. A leather-like synthetic resin sheet is generally called "artificial leather". Conventional artificial leathers are mainly prepared by sticking a surface layer of a foamed synthetic resin onto a basic fabric layer made of fibers and integrating them with each other.
The reason of requiring the basic fabric layer for preparing the conventional artificial leather is that the use of the foamed resin layer only cannot impart the properties, in particular, the tensile strength comparable to a leather to the artificial leather.
Due to the use of the basic fabric layer, a conventional artificial leather has a problem that the basic fabric layer peels off from the resin layer and also has a disadvantage that the production step becomes complicated.
Hitherto, as the synthetic resin for making artificial leathers, polyvinyl chloride series resins and polyurethane series resins have been mainly used since these resins show proper flexibility and can be easily foamed. Also, a polyamide resin was considered possible to use as a synthetic resin for an artificial leather but since it was difficult to foam the polyamide resin, the resin had not been widely used for artificial leathers. Other resins were originally considered to be unsuitable for making artificial leathers or it was difficult to foam these resins and, hence, these resins have never been used for the production of artificial leathers.
On the other hand, PAT has been once formed into fibers, compounded and then used for the production of an artificial leather in place of a basic fabric layer. However, PAT has never been used as a resin layer for an artificial leather since it is difficult to foam PAT. More specifically, the reason is that since PAT is hard, poor in flexibility and crystalline as well as has the characteristic that PAT suddenly changes from a solid to a liquid having a low viscosity at heating, the temperature range showing a viscosity suitable for foaming is narrow, thereby the temperature control at foaming is difficult and also the resin immediately releases gases which become the motive power for foaming, which results in making it difficult to foam the resin.
However, it is known that when PAT containing a foaming agent is extruded in the air from an extruder, it is foamed although the expansion ratio is low. The expansion ratio is usually no higher than twice. When the resin is extruded in a sheet form, the resin can be foamed at a relatively large expansion ratio but even in such a case, the expansion ratio is about 4 times. Thus, it has been attempted to foam PAT at a larger expansion ratio.
JP-A-55-2045 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") describes a process of foaming at a high expansion ratio by extruding PAT containing a foaming agent in a region having a reduced pressure. According to the process described in this patent, it is reported that a foamed material of a circular cross section foamed at an expansion ratio of from 22 times to 32 times was obtained. However, the process has a fault that it is difficult to construct a reduced pressure zone. In particular, it is difficult to undergo this process when it is intended to make an artificial leather. The reason is as follows. That is, since when PAT is extruded in a sheet form, the extruded sheet has an uneven surface by foaming and sometimes is largely deformed by foaming, it is not easy to construct a reduced pressure zone capable of continuously passing therethrough the sheet having such an irregular form and in particular, it is difficult to achieve sufficient reduced-pressure sealing for a sheet form material.
On the other hand, it is known that when a foamed plate made of a styrene series resin is bent, the foamed plate becomes rich in flexibility as described in JP-B-63-60705 (the term "JP-B" as described herein means an "examined published Japanese patent application"). However, the resin to be used in this patent is limited to a styrene series resin and the patent publication teaches nothing about a foamed plate of PAT. Also, the patent publication teaches only that the flexible foamed plate thus obtained is suitable for a material being used by bending as a cushioning material, a heat insulating material, or a packing material and teaches nothing about a material becoming a leather.
The present inventors have attempted to make an artificial leather by foaming PAT which was generally considered reluctant to foam and tried to foam PAT by extruding PAT in the air. As the result, it has been found that when PAT is extruded under a proper condition, a sheet foamed at about 4 times is obtained. Furthermore, the inventors have discovered that when the foamed sheet thus obtained is heated by being brought into contact with steam, etc., the sheet is further expanded to become a sheet uniformly and finely foamed at a higher expansion ratio.
Also, the inventors have further discovered that when the uniformly and finely foamed PAT sheet thus obtained is compressed by holding the sheet between rolls at normal temperature, the sheet increases the flexibility, whereby a leather-like flexible, high tensile strength, high tear strength and tough sheet is obtained. Furthermore, the inventors have confirmed that when the above-described uniformly and finely foamed PAT sheet is bent or crumpled in place of compressing by holding the sheet between rolls, a leather-like sheet is similarly obtained.
Also, as the result of investigating the structure of the leather-like sheet obtained as described above, it has been confirmed that the leather-like sheet is uniformly and finely foamed as well as the cell walls are in a specific state and also the whole sheet has a specific flexibility.
In this case, the term "uniformly and finely foamed" means that the cells are small ones having a diameter of substantially 1 mm or shorter and the cell walls are in a state having a thickness of not thicker than 100 .mu.m.
Also, the term "the cell walls are in a specific state" means that at least part of the cell walls extends while irregularly bending. Moreover, the term "the whole sheet has a specific flexibility" means that the sheet has a small value of flexibility by the Gurley flexibility test of no higher than 20,000 mg. Furthermore, it has been found that when the leather-like sheet is immersed in a strong alkali solution, the cells become continuous to render the sheet breathable and water absorptive.
The present invention has been made based on the above-described knowledges.